1. Field of the Invention
This invention relates to an image forming apparatus provided with an endless belt for transferring a sheet to an image forming means, or an endless belt on a surface of which an image to be transferred to a sheet is formed.
2. Description of the Related Art
As shown in FIGS. 1(A) and 1(B), an image forming apparatus adapted to fix a sheet 13 on a surface of an endless belt 12 passed around rolls 10, 11, and transfer toner images formed on photosensitive drums 14-17 onto the sheet 13 has already been known. In this image forming apparatus, the toner images are transferred successively from a plurality of photosensitive drums 14-17 onto the sheet 13 in accordance with a movement of the endless belt 12. Consequently, a multi-color toner image 18 is obtained on the sheet 13.
As shown in FIGS. 2(A) and 2(B), an image forming apparatus adapted to transfer a toner image 19 from a photosensitive drum 20 to a surface of an endless belt 12 as an intermediate transfer member, and then transfer this toner image 19 again to a sheet 13 has already been known as well. In this image forming apparatus, the endless belt 12 passes through a nip provided between a backup roll 21 and a bias transfer roll (not shown), and the sheet 13 also passes the same. During the passage of the endless belt and sheet through the nip, the toner image 19 on the endless belt 12 is transferred to the sheet 13 owing to an electric field generated by the bias transfer roll.
In these image forming apparatuses, the belt does not always move at right angles to the rolls 10, 11 due to manufacturing errors of the endless belt support rolls, errors of parallelism of the rolls, manufacturing errors of the belt, and a difference between the tension applied to the belt at one end portion of each roll and that applied to the belt at the other end portion thereof. Namely, the endless belt 12 moves in a twisted manner in the shape of the numeral "8" in plan in some cases as shown in the drawings. Consequently, the image transferred from the photosensitive drum is distorted.
For example, even when a rectangular image is tried to be formed in the apparatus of FIGS. 1(A) and 1(B), a rhomboidal image is transferred to the sheet 13 on the endless belt 12. Therefore, even when a skew of a lead edge LE of the resultant toner image 18 is small, that of a side edge SE is large as shown in FIG. 1(C).
Even when a rectangular image is tried to be formed in the apparatus of FIGS. 2(A) and 2(B), a rhomboidal image is transferred to the endless belt 12. In the condition shown in FIG. 2(A) in which the transferring of a toner image has just been done, a skew of a side edge only of the toner image 19 is large. However, after the toner image 19 has been turned around the roll 11, a lead edge thereof is also skewed. As a result, both the skew of the lead edge LE and that of the side edge SE are large as shown in FIG. 2(C) in the toner image 19 formed on the sheet 13.
In recent years, the improvement of the quality of images has been demanded even in an image forming apparatus utilizing electrophotographic system, such as a copier, and a permissible range with respect to such a skew of image has been severely limited. Therefore, developing the techniques for preventing such an inclined movement of a belt as mentioned above have been demanded.
The techniques for controlling a lateral movement of an endless belt include, for example, those disclosed in Japanese Patent Laid-Open No.110229/1997. However, even when a lateral movement of the endless belt is controlled properly, an inclined movement thereof with respect to the rolls cannot be prevented. It is also possible that the belt in a steady moving condition moves as it is left distorted with respect to the rolls 10,11 as shown in FIGS. 1(A), FIG. 1(B), FIG. 2(A) and FIG. 2(B). Therefore, the conventional techniques do not satisfactorily reduce the inclination of the image formed on a sheet.